To assess the performance of a number of processes in which urea or ammonia are employed, it is often necessary to have the ability to determine how much ammonia is in a process or effluent stream. Traditionally, samples have been extracted an analyzed by chemiluminescent techniques, resulting in extended lag times between sample acquisition and analysis. These methods also lacked a desired precision at low concentrations because they relied upon assumptions to make the necessary calculations.
Recently, interest has turned to tunable diode laser (TDL) absorption spectroscopy. In a typical TDL installation, the transmitter and receiver are mounted directly to the ductwork. Since ammonia can convert to other compounds or be absorbed in ash at temperatures less than 500° F., the ideal installation location for a TDL is upstream of the air heater. Installations of this type are difficult for a variety of reasons. Access is frequently difficult due to limited space existing between the economizer or catalyst outlet and the air heater inlet. Walkways are frequently absent. TDL's rely upon the line of sight between the transmitter and receiver, with a longer distance aiding accuracy at low concentrations. However, as the distance increases, the required alignment of the device becomes more difficult. Thermal cycles can also affect this alignment. Ash loading degrades the signal and in high ash environments, a longer TDL path length may not be possible. Maintenance is frequently difficult due to the instruments installed, location and the challenges with access.
TDL's measure the ammonia concentration on a line of sight path between a transmitter and a receiver. If the gas flow is stratified and the ductwork is large, this ammonia concentration may not be indicative of the actual, overall ammonia concentration. For this reason, on larger units or units with multiple ducts, a typical TDL installation may require more than one transmitting and receiving unit.
Many commercial processes use extractive techniques to obtain samples of off-gas from the exhaust. The extracted gas is typically cooled and then analyzed using mass spectrometry or non-dispersive infrared absorption methods or chemical cells. However, the steps required to obtain a sample of the off-gas from extractive techniques can result in time delays in acquiring the data. By contrast, a process using real time sensors could obtain selective measurements of the off-gas constituents and provide adjustment of the inputs to a furnace on a continuous feedback loop. In one example of a continuous extractive analysis, WO 97/499979 to Frish, et al., describes a TDL system to monitor trace concentrations (e.g., on the order of one part per million) of ammonia in gases extracted from coal-fired utility boilers. The system includes a filter to remove particulates and a heater and temperature sensors that maintain the temperature of extracted gases. The device illustrated employs a Herriot cell to magnify TDL sensitivity in a small foot print, but this is not an ideal arrangement for use in obtaining accurate readings from a utility boiler where gas (and particulate) component compositions can vary across any given cross section.
Both selective noncatalytic NOx reduction (SNCR) and selective catalytic NOx reduction (SCR) processes used for controlling nitrogen oxides release from power plants and other combustors, employ ammonia either directly or indirectly as a NOx-reducing reagent. It must be fed at the right concentrations and temperatures with regard to the NOx concentration to assure effective NOx control without excessive ammonia slip. There is always a delicate balance, and control systems must have accurate information to assure effective operation to comply with all regulations and guarantees as well as to avoid the practical problems of ammonium bisulfate production.
What is needed is a system designed to solve the difficulties of a typical TDL installation. A useful system would be capable of sampling gases over a broad array of locations through probes designed and operated to assure that extracted gas samples can be taken that are representative of actual operation conditions.
There is a present need for a process, apparatus and system that will enable the real-time analysis of ammonia concentration in a process or effluent stream.